1. Field of the Invention
The present invention is directed to an apparatus for stimulating living tissue, such as an implantable heart pacemaker, including means for insuring that a predetermined amplitude of the stimulating pulse is maintained over time.
2. Description of the Prior Art
An apparatus for stimulating living tissue, in the form of an implantable heart pacemaker, is disclosed in European application 0 071 965, which includes a battery having an internal resistance, which is dependent on the charge of the battery. A stimulating pulse generator is connected to the battery and includes an output capacitor, which is relatively slowly charged by the battery. To deliver a stimulating pulse, the capacitor is quickly discharged across the tissue to be stimulated. A measuring device is provided which measures the voltage across the output capacitor. A control device selects a predetermined value for the stimulating pulse amplitude. The control device controls the stimulating pulse generator so as to enable release of a stimulating pulse dependent on a predetermined stimulating pulse time interval, as well as on whether the voltage across the output capacitor, measured by the measuring device, has reached the predetermined value for the stimulating pulse amplitude.
In this known pacemaker, the stimulating pulses are separated by defined stimulating pulse time intervals. The pacemaker is provided with a battery for charging an output capacitor, which, for delivering a stimulating pulse, is discharged across the heart tissue. The internal resistance of the battery increases with time, which in turn increases the charging time of the output capacitor. In order to guarantee that a stimulation is always carried out with efficient stimulating pulse amplitude, the voltage across the output capacitor is measured and a stimulating pulse is not delivered until a defined minimum voltage has been reached. This means that the stimulating pulse is delayed if the output capacitor is not charged to the minimum voltage at the time when the stimulating pulse should have been delivered according to the defined stimulating pulse time interval. This delay results in a loss of control of the stimulating pulse time interval as it will be the charging time of the output capacitor that will determine the stimulating pulse time interval, and the charging time may vary. In particular, if the stimulating pulse amplitude and/or the stimulating pulse duration varies, the charging time of the output capacitor will vary.